A conventional fiber optic communication terminal is disclosed, for example, by Shibutani et al. in "Wide area video monitoring systems with subcarrier multiplex optical multi-access technique," Fall Meeting Issue B-711 of The institute of electronics, information and communication engineers 1992 which is incorporated herein by reference.
In the case of the conventional fiber optic communication system using this fiber optic communication terminal, light signals from fiber optic communication terminals are mixed, which are finally sent to the center station as one light signal. Although the light signals from the fiber optic communication terminals are mixed to be one light signal, since each fiber optic communication terminal modulates the light signal by an electric signal individually differing in the carrier frequency, it is same as when multiplexed by frequency division on the mixed light signal. Therefore, at the center station, since the frequency of the carrier assigned to each fiber optic communication terminal in known, the signals from the individual fiber optic communication terminals can be distinguished on the frequency axis of the received electric signal. Or when modulated by spread spectrum method, it is possible to distinguish the signals from the individual fiber optic communication terminals by demodulating accordingly at the center station.
In such constitution, however, if the wavelengths of light signals from the fiber optic communication terminals are close to each other, as the light signals interfere, beat interference occurs, which may adversely affect the signals, Generally, when two light signals interfere, and they are converted into electric signals, a signal of differential frequency of the two light signal frequencies is generated, and it is called beat interference or beat noise. If the frequency of this beat interference overlaps with the band of modulated signal, the modulated signal is influenced as a matter of course. Therefore, to prevent occurrence of this beat interference, it is necessary to set apart the wavelength of each light signal from each other. The separating distance of wavelength may be set so that beat interference may take place at a much higher frequency than the band of the modulated signal. For this purpose, the problem is that the light source must be selected so that the wavelengths of light sources of all fiber optic communication terminals be mutually different by more than a specific value.